Such arrays of transducers are used as phased arrays for focusing the sensitivity of the array in a desired direction. Preferably, the array should be usable in a broad frequency range. Phased arrays are usable as receiving arrays, eg for locating a signal source or for producing a two-dimensional image of one or more point sources or distributed sources, or for selecting signals from a particular source and excluding or attenuating signals from other sources. Phased arrays are also usable as transmitting arrays, eg for target illumination with projected beams. Signals that can be handled, ie received or transmitted, by such arrays are wave-energy signals having wavelengths that are comparable to the dimensions of the array and/or to the distances between individual transducers in the array.
Examples of such wave energy are sound energy within the audible frequency range or infrasound or ultrasound, which are outside the audible frequency range. In case of sound energy, receiving transducers are referred to as microphones, and transmitting transducers are referred to as speaker transducers. Another example of wave energy is electromagnetic energy such as radio frequency (RF) energy that can be received or emitted by suitable antennas eg for mapping the RF landscape or for focusing on a fixed or moving source or target.
With a given number of transducer elements, ie sensors or emitters, in the array, it is often an objective when designing the array to obtain a non-redundant distribution of the transducer elements, and at the same time to obtain a broad usable frequency range, good suppression of side lobes and near circular symmetry. Circular symmetry is also referred to as rotational symmetry and means that through rotation of a fraction 1/n, where n is an integer, of 360 degrees the array will cover it self or be in an identical position. Non-redundancy means that no spacing vector between any two transducer-elements is repeated. A non-redundant array has the advantage that with the given number of elements the maximum number of distinct lags is sampled. Thus, a non-redundant array provides a near optimum array design with respect to spatial sampling characteristics of the array.
The maximum side lobe level in the beam pattern of an array is a measure of its ability to reject unwanted signals and noise and to focus on particular propagating signals. It is therefore important to achieve good side lobe suppression for the array.
Circular symmetry of the array is desirable, because otherwise the source map resolution or a projected beam tends to be azimuth angle dependent.
Prior art arrays have been designed in seeking to meet the above-mentioned requirements including irregular arrays such as random arrays and logarithmic spiral arrays.
U.S. Pat. No. 5,838,284 discloses an array of transducers arranged on a single logarithmic spiral having several turns.
U.S. Pat. No. 6,205,224 discloses a circularly symmetric planar array. Its transducer elements are arranged on a plurality of identical logarithmic spirals at locations where the spirals intersect concentric circles of specified diameters.
When carefully designed such arrays are fairly successful in meeting the requirements. However, due to their complicated geometry they are difficult both to manufacture and also to operate. Also, the need for high resolution in the far field can only be met with relatively large dimensions of the arrays. Thus, an array with a diameter of several meter is often required. In connection with outdoor applications it is therefore of practical importance that the array construction allows for easy assembly and disassembly at the site of use, and for easy transport.
It is the object of the invention to provide a planar array with a simple geometry, which, without compromising non-redundancy, circular symmetry or well-controlled side lobe suppression, allows easy manufacturing and operation.